1. Field of the Invention
The present invention relates to electrostatic tomography.
2. Description of the Related Art
Many operations involve searching for human activity where the humans do not communicate and cannot be readily seen. For example, rescuers search for survivors in building rubble or in landslides caused by earthquakes, floods, and combat. As another example, tactical units for armed services and police have a need for information on a number or deployment of concealed hostile forces and if the concealed forces possess weapons, such as small arms, or some combination of this information. These hostile persons can be concealed within a building, a building complex, an extent of forest, an expanse of shrubs, or the like.
Unfortunately, very few sensor technologies effectively image the interior of building structures or densely foliated expanses. Infrared, acoustic and radiation systems are often ineffective.
Infrared sensors measure thermal radiation. Most building and earthen structures provide high thermal impedance, i.e., resistance to the flow of heat. Building material is often selected for its high thermal impedance to provide the thermal insulation property that is a basic function of many human shelters. High thermal impedance retards the flow of heat from humans touching the interior surface of exterior walls, through the exterior walls, to the exterior surface where hot spots can be detected by infrared sensors. Thus, infrared sensors are ineffective where the humans are in interior chambers, or are deeply buried, or in circumstances where humans, who are in contact with the inside surfaces of exterior walls, are also moving.
Acoustic sensors are ineffective for similar reasons. Most earthen or building structures also provide high acoustic impedance, which channels or dampens acoustic signals, or both. Thus, acoustic sensors are often ineffective, where the humans are in interior chambers, or are deeply buried.
Radiation systems are ineffective for several reasons. For example, high frequency electromagnetic radiation used in RADAR (radio detection and ranging) systems do not effectively penetrate buildings. Very high frequency, high energy radiation, such as X-ray systems, gamma ray systems, and neutron systems either penetrate the humans of interest nearly as effectively as the walls, giving very low contrast signals that are difficult or expensive to process, or produce so much ionizing activity so as to be harmful to the humans being detected. A system that injures the humans to be detected renders the system useless for search and rescue, for discriminating friendly forces from hostile forces or for situations, such as in an urban surrounding, where non-combatants may be exposed to excessive levels of ionizing radiation.
Similar concerns apply where the targets of interest are other biological entities, such as animals used for guard or attack purposes or contraband animals subject to illegal trade.
Low frequency electric fields with long wavelengths are known to penetrate various materials to distances related to their wavelengths and are known to be affected by the electrical properties of the material penetrated. (See, for example, Scharfetter H, Riu P, Populo M, Rosell J., “Sensitivity maps for low-contrast-perturbations within conducting background in magnetic induction tomography (MIT),” Physiol Meas, vol. 23: p195-202, 2002, which is incorporated by reference herein in its entirety). Within such distances, the electric fields can be considered “electrostatic” fields, and are so called hereinafter. In some references, such fields are sometimes called “quasi-electrostatic” fields.
Some techniques are known for inferring the distribution of electrical properties inside a region from electrostatic properties measured on a boundary of the region. These techniques are called hereinafter, “electrostatic tomography” techniques.
Measurements of electric field perturbation are used in a number of applications that do not require inversion using tomographic techniques. For example, electric wall stud locaters use a type of electric field measurement to sense differences in electrical capacitance to distinguish wood from air or insulation a few centimeters behind plasterboard. These single pole measurement devices do not estimate location or electrical properties of hidden materials, but merely detect changes in those electrical properties. In electric field intrusion detection systems, changes in electric fields are used to indicate the presence of an intruder by disruption in an electric field, but these systems do not use a plurality of independent measurements to estimate in three dimensions (3-D) the location or other properties of the intruder.
Intrusion systems are point sensors or netted point sensors. Their measurement data lacks the spatial and geometric information required for a tomographic inversion to reconstruct an image based on the intruder and surrounding space. Hence, the output of capacitance intrusion sensors is an indication that an intruder has perturbed the field at a particular location, usually by direct contact with the field or being in very close proximity to the field. For example, one form of capacitance based intrusion detection system uses a series of wires strung along a perimeter. When a conducting object, such as a person, approaches the wire assembly, some of the field lines emitted by the current carrying wire are intercepted by the proximate conductor and directed or shunted to ground, causing a corresponding decrease in the current measured in the sense wire. Localization of the disturbance may be accomplished by using time domain techniques widely known in the art. There is no practically feasible way a system like this could be used to distinguish proximate intruders from other conducting objects.
Another use of proximate electric field imaging disclosed in the literature involves detecting occupant proximity to seats for activating air bags and for child safety seats (see Gershenfeld et al U.S. Pat. No. 6,066,954 and Jinno, K., Ofuji, M., Asito T., and Sekido S. “Occupant Sensing Utilizing Perturbation of Electric Fields,” in Anthropomorphic Dummies and Crash Instrumentation Sensors (SP-1361), Society of Automotive Engineers (SAE), Warrendale, PA, pp 117-129, 1997, which are incorporate by reference herein in their entireties).
There are four principal approaches to electrostatic tomography currently practiced, distinguished by the property mapped inside the region:
1) electrical capacitance tomography (ECT) maps electrical permittivity;
2) electrical impedance tomography (EIT) maps electrical impedance;
3) electo-magnetic tomography (EMT) maps magnetic permeability; and
4) electric field tomography (EFT) maps displacement currents induced in a conductor.
ECT has been used to determine properties of fluid flow through pipes. The capacitance is measured between two or more electrodes attached to opposite sides of the pipe. These systems typically require electrical contact between the sensor electrodes and the item for which capacitance is to be measured. Such a system is described, for example, in U.S. Pat. No. 6,577,700 B1 to L. Fan and W. Warsito (hereinafter Fan), which issued Jun. 10, 2003, which is incorporated herein by reference in its entirety. These systems rely on the dielectric properties of the material flowing in the pipe to develop an approximate space-time distribution. These systems do not detect, locate or characterize target objects, including biological organisms, cached in a large structure, such as a building.
Electrical impedance tomography (EIT) has been used in medical applications, for example to determine, non-invasively, broken bones within flesh. Such systems involve electrical contact between sensors and the flesh surrounding the bone. Such systems have been applied only over distances from a few to a few tens of centimeters and rely on measurement of conductivity and not capacitance through living tissue. These systems do not detect, locate or characterize target objects, including biological organisms, cached in a large structure, such as a building.
Similarly, electro-magnetic tomography (EMT) applications involve sensor electrodes in contact with the subject and ranges of tens of centimeters for computing an approximate 3-D distribution of neuronal activity within a human brain from extra-cranial measurements of electric potential (EEG) and/or magnetic field (MEG). EMT produces a blurred-localized image of a point source resulting in a low-resolution image of brain activity during epileptic spike and other neurological events. EMT systems presuppose electrode contact with the scalp and exploit passive electromagnetic emissions from the brain. They use only receiving electrodes, not emitting electrodes. These systems do not detect, locate or characterize target objects, including biological organisms, cached in a large structure, such as a building.
Electric Field Tomography (EFT) uses measurements of electrical potential or displacement currents induced by changes in electrical potential to reconstruct the location, size, shape and orientation of proximate conducting objects. Some EFT systems are directed to measure relative position and orientation of a human hand for use as a computer interface device. Such systems are described in U.S. Pat. No. 5,844,415 to Neil Gershenfeld and Joshua R. Smith, which issued Dec. 1, 1998 (hereinafter Gershenfeld I); U.S. Pat. No. 5,914,610 to Neil Gershenfeld and Joshua R. Smith, which issued Jun. 22, 1999 (hereinafter Gershenfeld II); and U.S. Pat. No. 5,936,412 to Neil Gershenfeld and Joshua R. Smith, which issued Aug. 10, 1999 (hereinafter Gershenfeld III), which are all incorporate herein by reference in their entireties. These systems use EFT over distances within a room and used fixed geometries for sensor placement. These systems are not suggested for circumstances of concern in the present invention, such as detecting humans concealed behind blocking material, like building structures or wooded areas, over distances on the scale of a building (e.g., about 5 m and more) when there is no access to the space being measured for placement of sensors.
Some electric field systems are directed to geo-prospecting by mapping perturbations in electric fields injected into the ground over wide geographic areas. These systems equate discontinuities in spatially separated measurements with inhomogeneities in the sub-surface geographic features that could be indicative of subterranean petroleum pockets. These systems are not properly called tomographic in that there is no computation of the inverse, as described in more detail in a later section.
One notional system (U.S. Pat. No. 5,206,640 to Esko Hirvonen and Juhani Ninivaara issued Apr. 27, 1993, which is incorporated by reference herein in its entirety) suggests using electrostatic fields to detect vessels, such as submarines, in narrow seaways, like straits and harbors. This system detects vessels as changes in measured currents due to conductivity differences between the vessel and seawater. However, this system relies on sensors that are embedded in the same conducting medium as the target, i.e., seawater, and that have fixed, unchanging geometries within the space being monitored. This system is not suggested for circumstances of concern in the present invention, such as detecting humans embedded in a non-conducting medium (air) and concealed behind non-conducting blocking material, like building structures or wooded areas, when there is no access to the space being measured for placement of sensors.
None of these systems account for the complexities in electrostatic field measurements caused by the presence of buildings and building materials with unknown building components. For example, no current EFT systems account for the impact on electrostatic fields of walls and metal conduits for electrical, water, and air ducts or the varying effects of wood, concrete and other construction materials that have unknown or only partially known distributions in a region of interest. Most ignore the environment and rely on temporal changes to distinguish differences from a fixed background state. Such systems would not work for detecting sleeping or stationary humans, for example.
These EFT systems also involve fixed and static geometries for sensors that make it difficult to adapt them to buildings of arbitrary shape and size and rapidly changing tactical situations. For example, U.S. published application No. US2002/0038096 by Gregory and Gregory published Mar. 28, 2002 (Gregory), which is incorporated by reference herein in its entirety, uses an array of sensors on a sensor holder that fixes the geometry of the sensors and therefore fixes the region to be measured. The use of fixed spacing between electrodes allows certain quantities in the inversion process to be pre-computed. Inversions using such pre-computed quantities cannot be performed for electrode spacing that is changing on tactical scales.
No systems exploit the properties of electrostatic fields to detect, locate and characterize one or more dielectric or conducting target objects, including biological entities, inside of at least partially unknown and variable building-sized regions.
Therefore, what is needed is a sensor technology that allows dielectric and conducting targets inside of building sized regions to be detected and characterized from the outside.